1. Field of the Invention
The present invention relates to a piezoelectric actuator in which an active portion is deformed when a voltage is applied.
2. Description of the Related Art
As a piezoelectric actuator, an actuator which has a plurality of stacked piezoelectric layers, a common electrode arranged between the piezoelectric layers, and a plurality of individual electrodes arranged on an upper surface of the uppermost piezoelectric layer has been known.
In such a type of piezoelectric actuator, the common electrode is formed by applying an electrode material on an entire surface of the piezoelectric layer. Moreover, terminals for the individual electrodes are provided on the uppermost piezoelectric layer, corresponding to the individual electrodes respectively. Furthermore, each of the terminals for individual electrodes is provided to be in an electrical conduction with one of the individual electrodes, and is electrically connected to a drive circuit for applying selectively a drive voltage to the individual electrodes, via wires formed in an FPC (Flexible Printed Circuit)
In such piezoelectric actuator, an area between each of the individual electrodes and the common electrode acts as an active area, and when an electric field is applied to the active portion, the active portion is deformed independently. In other words, by the drive voltage being applied to the individual electrode from the drive circuit via the terminal for the individual electrode (hereinafter, ‘individual electrode terminal’), an electric field is generated between the individual electrode and the common electrode, and the active portion between the individual electrode and the common electrode is deformed (refer to Japanese Patent Application Laid-open No. 2005-27403). In the piezoelectric actuator having such structure, a high densification of the individual electrodes has been sought to reduce a size of the piezoelectric actuator. However, when the individual electrodes are arranged highly densely, at the time of deformation of the active portion, a problem of cross-talk, in which a vibration of the active portion affects an adjacent active portions, becomes remarkable.
The common electrode in the piezoelectric actuator described in Japanese Patent Application Laid-open No. 2005-27403 is formed by applying an electrode material on the entire surface of the piezoelectric layer. Coefficient of linear expansion of the electrode materials forming the common electrode in the piezoelectric actuator described in Japanese Patent Application Laid-open No. 2005-27403 is different from a coefficient of linear expansion of the piezoelectric material such as PZT (lead zirconium titanate) forming the piezoelectric layer. Accordingly, the piezoelectric layer might bend by warping at the time of baking, or the piezoelectric layer might not bend but a residual stress after baking might increase.
By forming the common electrode described in Japanese Patent Application Laid-open No. 2005-27403 on the entire surface of the piezoelectric layer, a floating capacitance which does not contribute to the deformation of the active portion is generated between the common electrode and the individual electrode terminal. Since the floating capacitance generates heat, if the capacitance becomes greater, an amount of heat generated by the piezoelectric actuator increases.